1. Field of the Invention
The present invention relates to an inversion spring and more particularly to an inversion spring contact driver of a thermal overload relay.
2. Description of the Related Art
Referring to FIGS. 5, 6, 7, and 8 which show a conventional thermal overload relay, FIG. 5 is a front view of a major part of the relay in the reset position, and FIG. 6 is a front view of the major part of the relay in the set position. FIG. 7 is a plan view of inversion spring unit of the relay. FIG. 8 is a side view of FIG. 7.
The contact unit 3 of the relay is disposed in the electrically insulating case 20 of the relay. The contact unit 3 comprises a movable contact support 3a having a coupling hole 3b at one end of contact support 3a. Contact support 3a is disposed to fit in groove 20a of case 20 so as to be slideable in the axial direction of the support. A normally-closed and a normally-open movable contact 3e and 3f are disposed on support 3a, respectively adjacent to normally-closed and normally-open fixed contacts 3c and 3d. The fixed contacts 3c and 3d are disposed in the case for selective contact with their respective movable contacts. A return rod 3h which is guided by the case so that the rod can be engaged with the projection 3g of the support to return the contact unit to the reset position. The inversion mechanism 4 of the relay comprises a see-saw-type release lever 4A pivotably supported in case 20 and engaged at one end of the lever with a shifter 2 and at the other end with an inversion spring unit 4B. Shifter 2 is moveable with a bimetal 1 and inversion spring unit 4B is engageable with the other end of the lever and is secured at the butt of the inversion spring unit to an inverting position adjuster 5. Position adjuster 5 is engaged in the coupling hold 3b of the movable contact support 3a at the free tip of the spring unit so that the posture of the spring unit is inverted by the swing of the lever. As shown in FIGS. 7 and 8, the inversion spring unit 4B comprises first and second springs 4B1 and 4B2, respectively. The first spring 4B1 is made of an oblong spring plate, and has a long and short tongue 4B11 and 4B12, respectively, that are formed by cutting and bending the central portion of the plate. Each of tongues 4B11 and 4B12 have free ends. The short tongue 4B12 can be moved through the opening 4B13 of the long tongue 4B11. The second spring 4B2 is shaped as a U, and has an arm 4B21 resiliently engaged with the edge of the first spring 4B1 at the opening 4B13 thereof as a movable joint. Second spring 4B2 has another arm 4B22 resiliently engaged with the end of the short tongue 4B12 as a movable joint. The inverting position adjuster 5 comprises a support member 5A which is obtusely bent and has a short lug 5A1 having a sharp-edged tip engaged in the V-groove 20b defining a fulcrum on the inside surface of the case 20. This arrangement allows position adjuster 5 to pivot about lug 5A1. Position adjuster 5 also includes a long lug 5A2 to which the fulcrum of the first spring 4B1 is secured. Adjusting screw 5B engaged in the tapped hole 5A3 of long lug 5A2 so as to be movable back and forth, and a compressed spring 5C is interposed between the intermediate portion of long lug 51A and the inside surface of the case.
When the release lever 4A of the inversion mechanism 4 is swung clockwise by the shifter 2, the tip of the lever pushes the short tongue 4B12 of the first spring 4B1 of the inversion spring unit 4B. When the short tongue 4B12 is thereby moved through the opening 4B13 in which the inversion dead point for the short tongue is located, the posture of the long tongue 4B11 is inverted by the second spring 4B2 so that the normally-closed movable contacts 3e of the contact unit 3 are disengaged from the normally-closed fixed contacts 3c, and the normally-open movable contact 3f of the contact unit are engaged with the normally-open fixed contacts 3d thereof, as shown in FIG. 6. In order to return the contact unit 3 into the original state after that, the return rod 3h is pushed down to move the projection 3g of the movable contact support 3a rightward as shown by a dotted line in FIG. 6. This causes the posture of the long tongue 4B11 to invert back beyond the dead point to return the first spring 4B1 to its original state as shown in FIG. 5. The position of the inversion dead point can be adjusted in a stepless manner by moving the adjusting screw 5B of the inverting position adjuster 5 back and forth with the use of a screwdriver to move the inversion spring unit 4B as a whole, depending on the force of the compressed spring 5C.
An inherent problem with the prior art device described above is that friction between the first and second springs displaces the dead point of the spring unit. Specially, the arms 4B21 and 4B22 of the second spring 4B2 of the inversion spring unit 4B, which functions as the contact driver of the conventional thermal overload relay, are resiliently engaged with the edge of the first spring 4B1 of the spring unit at the opening 4B13 of the first spring and with the tongue 4B12 thereof, respectively, causing friction at the points of the engagement of the first and second springs. Another problem with the prior art device is that since the first and second springs 4B1 and 4B2 need to be engaged with each other, it is difficult to automatically assemble the inversion spring unit.
The present invention overcomes the problems of the prior art by providing an improved inversion spring unit for a thermal overload relay that has a simple construction, is easy to assemble, and whose inversion dead point is kept in a fixed position.